One of the critiques of MRI has been that the images are plagued by motion caused artifacts. Artifacts due to motion appear as "ghosting" or smearing due to spin dephasing from view to view and/or a loss of signal due to the movement of spins in a single view. The artifacts due to motion become particularly severe when the time to echo (TE) values are long.
As can be expected a plethora of techniques have been offered as solutions to the motion artifact problem in body imaging. Among the prior art techniques for suppressing the motion caused artifacts have been gating techniques, rendering of the phase encoding pulse amplitudes, fast imaging techniques, and/or scan sequences that are insensitive to motion.
In general then, stationary spins are refocussed at the apex of the echo but the moving spins are not refocussed with the stationary spins because of the changing locations. Accordingly, there is spin dephasing due to motion.
None of the prior art techniques has been completely successful in removing the motion caused artifacts. For example, gating techniques require costly instrumentation, are time consuming and do nothing to correct dephasing that occurs in a single view. Further, gating limits the use of variations in such normal variables as TE or TR because the sequence is tied to the gating time.
Fast imaging techniques, such as "one-breath hold" techniques do not correct for dephasing that occurs in a view. In general, while fast imaging reduces motion artifact by reducing the sequence time, they are not nearly fast enough to prevent in-view artifacts.
The reordering techniques only work on periodic type motion, however, body motion is not periodic or only periodic as a first order approximation.
The prior art motion insensitive scan sequences use large rephasing gradients to eliminate or reduce the signal inconsistencies of the moving spins. The rephasing gradients are determined by solving equations designed to set the phase of the spins to zero at the apex of the signal.
The phase shift of a spin subjected to a gradient varying in time is given by: ##EQU1## where:
.gamma. is the gyromagnetic constant,
G(t') is the gradient amplitude at time t, and
X(t') is the position of the spin at the time t.
The position X(t) may be analyzed by expansion in a Taylor Power Series, thus: EQU X(t)=X(o)+X'(o)t/1!+X"(o)t.sup.2 /2!+X'"(o)t.sup.3 /3!+. . . X.sup.[n] (o)t.sup.n /n! (2)
or ##EQU2##
Accordingly by combining equations (1) and (2), the phase is given by: ##EQU3##
By setting this equation equal to zero, values for the gradients to give zero phase shift for spins in motion can derived and solved. See for example, the article "Motion Artifact Suppresion Technique (MAST) for MR Imaging" by P. M. Pattany et al in the Journal of Computer Assisted Tomography, Vol. 11(3) pp 369-377 May/June 1987. The gradients thus obtained will provide a motion insensitive scan sequence.
However, there are problems with this technique. Among the problems are that the gradients described by the solution to the equation are relatively large and have finite rise times and therefore, cause eddy current problems. In addition, the tailoring and timing of the gradients has to be practically perfect to obtain the zero phase shift desired, such perfection is not easy to achieve. Still further, the use of the large gradient pulses tends to increase the time to echo TE. Thus a minimum time to echo problem is created.
Accordingly, those skilled in the art are continuing to search for methods and apparatus to obtain motion insensitive data in magnetic resonance imaging systems without the necessity of using rephasing gradients that cause the eddy current problems and require accurate tailoring and timing.
A related objective is to reduce view to view motion caused artifacts and to practically eliminate motion caused loss of signal that occurs in a single view.